Removal of hydrogen sulfide and sulfur recovery from a gas stream by catalytic direct oxidation and claus reaction

ABSTRACT

A process for the removal of hydrogen sulfide and sulfur recovery from a H 2 S-containing gas stream by catalytic direct oxidation and Claus reaction through two or more serially connected catalytic reactors, wherein a specific control of the oxygen supplement is operated. The control and improvement of the process is obtained by complementing, in each major step of the process, the H 2 S-containing gas stream by a suitable flow of oxygen, namely before the H 2 S-containing gas stream enters the Claus furnace, in the first reactor of the process and in the last reactor of the process. Especially in application in a SubDewPoint sulfur recovery process the H 2 S/SO 2  ratio is kept constant also during switch-over of the reactors R1 and R by adding the last auxiliary oxygen containing gas directly upstream the last reactor R so that the H 2 S/SO 2  ratio can follow the signal of the ADA within a few seconds.

The present invention relates to a process for the removal of hydrogensulfide and sulfur recovery from a gas stream by catalytic directoxidation and Claus reaction. More specifically, the present inventionrelates to the control and the optimization of a process for the removalof hydrogen sulfide and sulfur recovery from a gas stream by catalyticdirect oxidation and Claus reaction.

BACKGROUND OF THE INVENTION

The presence of sulfur in industrial gases causes significantenvironmental problems, and therefore, strict requirements are in placeto remove sulfur from gas streams, in particular in petroleum refineryand natural gas plants but also in biogas plants, from H₂S scrubbers,etc.

Sulfur recovery units are thus installed to convert poisonous sulfurcompounds, as H₂S into harmless elemental sulfur.

A widespread method for desulfurization of sulfur-containing gasstreams, in particular from gas streams in petroleum refineries andnatural gas plants is the Claus process. The Claus process is long-knownand operates in two major process steps. The first process step iscarried out in a furnace where hydrogen sulfide is converted toelemental sulfur and sulfur dioxide at temperatures of approximately 900to 1400° C. by the combustion of about one third of the hydrogen sulfidein the gas stream. The so obtained sulfur dioxide reacts with hydrogensulfide in the furnace to elemental sulfur. Thus, in this first step ofthe Claus process, about 60 to 70% of the H₂S in the feed gas areconverted.

To achieve higher sulfur recovery rates two to four catalytic stepsfollow where the Claus reaction according to Eq. 1:

2H₂S+SO₂

3/xS_(x)+2H₂O  Eq. 1

continues.

A known process, in which H₂S and SO₂ are converted to elementarysulfur, with good desulfurization efficiency, is, for example, a Clausprocess with four serially connected catalytic reactors. To furtherincrease the desulfurization efficiency of over 99% Claus process withfour serially connected catalytic reactors with the last two beingoperated below the sulfur dew point. As a consequence, the chemicalequilibrium is shifted more strongly in the direction of reaction of H₂Sand SO₂ to elementary sulfur than in a conventional Claus process inwhich temperatures are not permitted to fall below the sulfur dew pointin any of the catalytic reactors.

The reason for this is that a major part of the formed sulfur is removedfrom the stream by adsorption on the catalyst and thus the equilibriumof the reaction of Eq. 1 is shifted by the sulfur elimination to theright side of the equation.

The catalyst is inactivated by the sulfur condensation so that it mustbe regenerated after a certain time. To maintain a continuous operationof the plant, of the four reactors the first is always operated as aconventional Claus reactor, one is regenerated, while the other two arerun below the sulfur dew point. For regeneration, the gas stream, forexample, is heated up by means of gas-gas heat exchangers so that bypassing the heated gas over the catalyst loaded with sulfur, the sulfuris evaporated.

If the sulfur loading of the catalyst reaches a certain level, anautomatic switching of the reactor to the regeneration phase and acorresponding switching of the two reactors working below the sulfur dewpoint take place.

Such a process is described, for example, in Oil & Gas Journal of Sep.12, 1983, on pages 156-160.

The known process has the great cost disadvantage that at least fourClaus reactors, of which two in each case are operated below the S dewpoint, are necessary to achieve an S recovery of over 99%.

An improvement of this process has been disclosed in U.S. Pat. No.4,957,724 which involves the use of two serially connected reactors,both reactors containing an active catalyst for the direct oxidation ofH₂S to sulfur, wherein the first reactor is operated above the sulfurdew point, and the second reactor is operated below the sulfur dewpoint. This process however involves drastic changes of the reactiontemperature between the two reactors and even between the two reactionzones of the reactors, which ends up being significantly cost effective.

In addition, the processes of the prior art fail to compensate thevariation of concentration in H₂S in the composition of theH₂S-containing gas feed, and the possible perturbations usuallyoccurring throughout the process such as a variation of the temperaturesin the reactors or loss of sulfur conversion due to partial deactivationof the catalyst, that lead to drift of the desulfurization efficiency.Indeed, when a sulfur recovery unit is operating, various operationalconditions can have an impact on the overall sulfur recovery.

There remains a need for a sulfur recovery process that could becontrolled in real time to avoid any drift of the desulfurizationefficiency and therefore improves the overall desulfurizationefficiency.

U.S. Pat. No. 2,919,976 for example focuses on the control of thetemperature in the different reactors of a Claus unit and discloses theintroduction of oxygen at the entry of the reactors in order to generateenough heat in the converters without having to use additional heaters.In this document, an H₂S/SO₂ controller can be used to control H₂S/SO₂volumetric ratio of the feed and introduce H₂S or SO₂ to keep theH₂S/SO₂ volumetric ratio constant in the feed. However, this documentfails to provide any solution to the drift of the desulfurizationefficiency that result from the competitive reactions that take place inthe reactors (hydrolysis of COS and CS₂, oxidation of H₂S, Clausreaction) and the variations of operational conditions during the wholeprocess. U.S. Pat. No. 2,919,976 only ensures that the gas entering theprocess has the proper H₂S/SO₂ volumetric ratio. This cannot guaranteethat this ratio will not vary during the different steps of the process.

U.S. Pat. No. 5,965,100 discloses a process for recovering sulfurinvolving a furnace operating with sub stoichiometric proportions ofoxygen to avoid sulfate formation that would poison the catalyst used inthe first reactor, and discloses the use of two additional rectorsoperating with specific catalysts having a spinel structure and with anair inlet. These special spinel structure catalysts favor oxidationreactions in the reactors. Therefore, U.S. Pat. No. 5,965,100 disclosesto control the H₂S/SO₂ volumetric ratio at the outlet of each reactorsand to adjust the air entering each reactors so as to keep the overalloxidation just below or at the stoichiometric level. It is therefore“expected” in U.S. Pat. No. 5,965,100 that the gas stream exiting theprocess will have a H₂S/SO₂ volumetric ratio of 2:1. However, aspreviously explained, it is not possible to anticipate that the H₂S/SO₂volumetric ratio will remain at the target value of 2:1 during the wholeprocess considering the various reactions that take place in thereactors and since variations in operational conditions will impact theequilibrium of each reaction. In addition, in U.S. Pat. No. 5,965,100,when a variation in the ratio is observed at the outlet of the secondreactor, air is adjusted at the entry of the furnace. This results in along time period (at least 5 minutes to hours depending on the size ofthe unit) before the adjustment of air in the furnace finally improvescorrectly the H₂S/SO₂ ratio in the second reactor. Finally, the unitdisclosed in U.S. Pat. No. 5,965,100 is costly since it involves atleast three H₂S/SO₂ controllers and specific catalysts.

U.S. Pat. No. 5,028,409 discloses a method for recovering sulfur fromgaseous materials containing H₂S involving a specific combustion zonecapable of burning pollutants such as ammonia. Each of the reactionzones contains a reheating unit in which air can be supplied to provideenough heat to the catalytic bed, a catalytic zone, a flow rate control,a temperature control and a H₂S/SO₂ ratio control. Document U.S. Pat.No. 5,028,409 intends to maintain the H₂S/SO₂ ratio constant during thewhole process. However, this document teaches to introduce air into thereheating units of each reaction zones and not directly in the reactors.When doing so, the air is burnt in the reheaters to provide heat to thereactor, and the remaining amount of oxygen available to react with H₂Sin the reactors cannot be precisely controlled. Here again, this priorart patent intends to maintain the H₂S/SO₂ ratio constant during thewhole process but provides no convincing solutions to do so. Inaddition, the resulting unit is incredibly costly considering all thecontrols of temperature, flow rate, H₂S/SO₂ ratio necessary in eachcatalytic zone.

Therefore, there remains a need in industry for a reliable controlledClaus process with a high operational availability and cheap inoperation. The process should in particular provide a very highdesulfurization efficiency (both conversion and selectivity) which isstable throughout the process, and that compensate any variation of theH₂S/SO₂ ratio at the outlet of the process with high reactivity, inparticular when the switch-over of the reactors is needed for catalystregeneration. These goals can be obtained thanks to the process of theclaimed invention.

The object of the present invention to provide a process for the removalof hydrogen sulfide (H₂S) from a H₂S-containing gas stream through twoor more serially connected catalytic reactors, which process comprises:

a) mixing the H₂S-containing gas stream with a main oxygen-containinggas stream to obtain a gas stream containing both H₂S and oxygen,

b) introducing the obtained gas stream containing both H₂S and oxygeninto a furnace whereby a gas stream depleted in H₂S is obtained,

c) transferring the gas stream depleted in H₂S to a sulfur condenser toobtain a gas stream depleted in sulfur,

d) introducing the gas stream depleted in sulfur, optionally togetherwith a first auxiliary oxygen-containing gas stream, into a firstcatalytic reactor R1 containing a catalyst system which catalyzes theClaus reaction of H₂S with sulfur dioxide (SO₂), the hydrolysis of COSand CS₂ and optionally direct oxidation of H₂S with oxygen to sulfur,said reactor being operated at a maximum temperature T^(R1) _(max)between 290 and 350° C., whereby a gas stream depleted in H₂S isobtained,

e) transferring the gas stream depleted in H₂S to a sulfur condenser toobtain a gas stream depleted in sulfur,

f) optionally introducing the gas stream depleted in sulfur obtainedfrom reactor R1 through a series of reactors and condensers, preferably1 or 2, each reactor containing a catalyst system which catalyzes theClaus reaction of H₂S with sulfur dioxide (SO₂) before reaching the lastreactor R of the process,

g) introducing the gas stream depleted in sulfur together with a lastauxiliary oxygen-containing gas stream into the last catalytic reactor Rcontaining a catalyst system which catalyzes the Claus reaction of H₂Swith sulfur dioxide (SO₂) and the direct oxidation of H₂S with oxygen tosulfur, said reactor being operated at a maximum temperature T^(R)_(max) below the maximum temperature T^(R1) _(max) of reactor R1,whereby a gas stream depleted in H₂S is obtained,

h) optionally transferring the gas stream depleted in H₂S to a sulfurcondenser to obtain a gas stream depleted in sulfur,

i) measuring the volumetric ratio of H₂S/SO₂ at the exit of the lastcatalytic reactor R,

wherein

the flow rate of oxygen in the main oxygen-containing gas stream and inthe optional auxiliary oxygen-containing gas streams represents 96 to99.9 vol. % of the total flow rate of the oxygen supplemented in theprocess, preferably 98 to 99.8 vol. %, and more preferably 98.5 to 99.5vol %

the flow rate of oxygen in the last auxiliary oxygen-containing gasstream represents 0.1 to 4 vol. % of the total flow rate of the oxygensupplemented in the process, preferably 0.1 to 2 vol. %, and morepreferably 0.5 to 1.5 vol. % and

wherein the flow rate of oxygen in the last auxiliary oxygen-containinggas stream is adjusted depending on the value of the volumetric ratio ofH₂S/SO₂ measured at the exit of the last catalytic reactor R in step i)so that the volumetric ratio of H₂S/SO₂ measured in step i) remainsbetween 1.9 and 2.2.

It is indeed of the merit of the inventors to have discovered that itwas possible to control and improve a Claus process by complementing, ineach major step of the process, the H₂S-containing gas stream by asuitable flow of oxygen. The oxygen implementation should at least beoperated before the H₂S-containing gas stream enters the Claus furnaceand in the last reactor of the process. It is indeed possible with suchcontrol to significantly reduce the period of time between themeasurement of a deviation from the optimum H₂S/SO₂ ratio of 2 and thereaction to this deviation by adjusting the oxygen both at entry of thesystem and in the last reactor. This provides a great reactivity to thewhole process and an improvement in sulfur conversion since the periodof time where the H₂S/SO₂ ratio is outside the optimum range of 1.9 to2.2 is significantly reduced.

Step a

In the first step of the process of the invention, a H₂S-containing gasstream (acid gas) is mixed with a main oxygen-containing gas stream toobtain a gas stream containing both H₂S and oxygen that will enter theClaus furnace.

In the furnace, the two following oxidation reactions of H₂S will takeplace:

2H₂S+3O₂→2SO₂+2H₂O

2H₂S+SO₂→3S+2H₂O.

The flow rate of oxygen in the main oxygen-containing gas stream and inthe optional auxiliary oxygen-containing gas streams represents 96 to99.9 vol. % of the total flow rate of the oxygen supplemented in theprocess, preferably 98 to 99.8 vol. %, and more preferably 98.5 to 99.5vol %.

The flow rate of oxygen in the main oxygen-containing gas stream and inthe optional auxiliary oxygen-containing gas streams, while alwaysrepresenting 96 to 99.9 vol. % of the flow rate of the totaloxygen-containing gas stream supplemented in the process, can preferablybe optimized by ensuring a volumetric ratio of H₂S in the H₂S-containinggas stream/O₂ in the main oxygen-containing gas stream be above or atthe stoichiometric value of the reactions operated in the furnace of 2,preferably from 2.002 to 2.5, more preferably from 2.002 to 2.2, andeven more preferably from 2.002 to 2.08.

Of course, when impurities that react with oxygen are present in theH₂S-containing gas stream (acid gas), flow rate of oxygen in the mainoxygen-containing gas stream should be adjusted by the skilled person sothe oxygen available to react with H₂S in the furnace remains below orat the stoichiometric value of the reactions operated in the furnace(corresponding to a volumetric ratio of H₂S in the H₂S-containing gasstream/O₂ in the main oxygen-containing gas stream of 2 or more).

This is preferably done by measuring the flow rate of H₂S in theH₂S-containing gas stream entering the system and defining the flow rateof oxygen in the main oxygen-containing gas flow so that the flow rateof oxygen in the main oxygen-containing gas stream be proportional tothe flow rate of H₂S in the H₂S-containing gas stream of aproportionality factor (a), said factor (a) being calculated so that thevolumetric ratio of H₂S in the H₂S-containing gas stream/O₂ in theoxygen-containing gas stream be above the stoichiometric value of thereactions operated in the furnace of 2 (corresponding to a maximum of 1mole of O₂ for 2 moles of H₂S).

The H₂S-containing gas stream entering the process of the inventionpreferably contains from 35 to 99.9 vol. % of H₂S, and preferably from40 to 99 vol. % H₂S, more preferably 50 to 98 vol. % H₂S.

The oxygen-containing gas stream preferably used in the process of theinvention is the air for obvious economic reasons, thus containingaround 20 vol. % of O₂ but could also be pure O₂.

Thus, for example if the H₂S-containing gas stream entering the processcontains 70 vol. % of H₂S, and the oxygen-containing gas stream contains20 vol. % of O₂, the proportionality factor (a) will be max 1.75 so thatthe oxygen-containing gas flow be max 1.75 times the H₂S-containing gasflow.

In a preferred embodiment of the present invention, in order to increasethe productivity of the Claus reaction in the furnace, the volumetricratio of H₂S in the H₂S-containing gas stream/O₂ in the mainoxygen-containing gas stream is maintained above the stoichiometricvalue of the reactions operated in the furnace of 2 during the wholeprocess, in particular between 2.002 to 2.5, preferably 2.002 to 2.2,more preferably 2.002 to 2.08.

Step b

Step b) of the claimed process involves the introduction of the obtainedgas stream containing both H₂S and oxygen into a furnace where H₂S isconverted to elemental sulfur and SO₂. The gas stream depleted in H₂Salso contains unreacted H₂S as well as impurities formed in the furnacesuch as COS and CS₂.

The furnace is preferably operated at a temperature of 900° C. to 1400°C., more preferably 1100° C. to 1300° C.

Step c

The claimed process further includes a step c) where the gas streamdepleted in H₂S leaving the furnace is cooled by passing through acondenser where liquid sulfur is condensed and withdrawn and a gasstream depleted in sulfur is recovered.

A gas stream depleted in sulfur is thus obtained, wherein preferably 50to 70 vol. % of the H₂S contained in the H₂S-containing gas streamentering the process is converted into sulfur.

Step d

The gas stream depleted in sulfur obtained in step c) will then beintroduced, optionally together with a first auxiliary oxygen-containinggas stream into a first catalytic reactor R1 containing a catalystsystem which catalyzes the Claus reaction of H₂S with sulfur dioxide(SO₂), the hydrolysis of COS and CS₂ and optionally the direct oxidationof H₂S with oxygen to sulfur.

Within the meaning of the invention, the first reactor is the firstClaus reactor of the process in which the H₂S-containing gas streamenters.

The gas stream depleted in sulfur obtained in step c) containsimpurities formed in the furnace such as COS and CS₂ which need to beeliminated. The first reactor R1 of the process of the invention aims atconverting a maximum amount of COS and CS₂ to H₂S by hydrolysis.However, hydrolysis of COS and CS₂ to H₂S can only be achieved when thecatalyst system of reactor R1 reaches a maximum temperature T^(R1)_(max) of 290° C. to 350° C. This temperature in reactor R1 can beachieved when the gas stream depleted in sulfur obtained in step c)reaches a temperature of about 220-250° C., depending on H₂S and SO₂concentration in vapor phase. When the gas stream at 220-250° C. reactswith the Claus catalyst, the exothermic Claus reaction is conducted,therefore increasing the temperature in the reactor to the desiredmaximum temperature of 290-350° C. ensuring hydrolysis.

In order for the gas stream depleted in sulfur obtained in step c) toreach a temperature between 220 and 250° C., it may be necessary tointroduce a heater between the condenser of step c) and the reactor 1 ofstep d) in order to pre-heat the gas stream depleted in sulfur obtainedin step c) up to a temperature of about 230° C.

Therefore, in a preferred embodiment, the gas stream depleted in sulfurobtained in step c) further passes through a heater located between thecondenser of step c) and the reactor 1 of step d).

To further increase the temperature of the gas stream depleted in sulfurobtained in step c) in a simple and economic manner, the process of theinvention introduces first auxiliary oxygen-containing gas stream intoreactor R1 to react with H₂S through direct oxidation according to Eq.2.

2H₂S+O₂

2/xS_(x)+2H₂O+heat  Eq. 2

This reaction produces the heat necessary to operate the reactor R1 atthe desired maximum temperature T^(R1) _(max) of 290 to 350° C.,preferably 310 to 340° C., and more preferably 315 to 330° C. so thatthe maximum amount of COS and CS₂ be eliminated by hydrolysis at thisstage.

In a preferred embodiment, the optional first auxiliaryoxygen-containing gas stream can also be introduced in a heater betweenthe condenser of step c) and the reactor 1 of step d) in order to bepre-heated.

The more oxygen is added to reactor R1, the more heat is producedthrough exothermic direct oxidation of H₂S.

However, reactor R1 should also mainly conduct the Claus reaction of H₂Swith sulfur dioxide (SO₂) so that the overall process of the inventionefficiently removes H₂S.

Therefore, the optional first auxiliary oxygen-containing gas streamshould only represent 0.1 to 19.9 vol. % of the total oxygen-containinggas stream supplemented in the process, preferably 0.5 to 9 vol. % andmore preferably 1 to 5 vol. % of the total oxygen-containing gas flowsupplemented in the process.

The optional flow rate of oxygen in the first auxiliaryoxygen-containing gas stream, while always representing 0.1 to 19.9 vol.% of the total oxygen-containing gas stream supplemented in the process,can be adjusted to provide more or less heat to reactor R1 in order toensure that the maximum temperature T^(R1) _(max) in reactor R1 remainsbetween 290 to 350° C., preferably 310 to 340° C., and more preferably315 to 330° C.

In a preferred embodiment of the present invention, in order to maximizethe amount of COS and CS₂ eliminated by hydrolysis and the H₂S removalthrough Claus reaction in the reactor R1, the maximum temperature T^(R1)_(max) in reactor R1 is maintained between 290 to 350° C., preferably310 to 340° C., and more preferably 315 to 330° C. during the wholeprocess.

If the maximum temperature T^(R1) _(max) measured happens to be below290° C., the flow rate of the optional first auxiliary oxygen-containinggas stream can be automatically increased. If the maximum temperatureT^(R1) _(max) is over 350° C., the flow rate of the optional firstauxiliary oxygen-containing gas stream can be automatically decreased.

In a preferred embodiment, the flow rate of oxygen in the firstauxiliary oxygen-containing gas stream at the entrance of the firstreactor is proportional to the flow rate of oxygen in the main auxiliaryoxygen-containing gas stream sent to the furnace of a proportionalityfactor (b). In this embodiment, the flow rate of oxygen in the mainauxiliary oxygen-containing gas stream is fixed to maintain both thevolumetric ratio of H₂S in the H₂S-containing gas stream/O₂ in the mainoxygen-containing gas stream above the stoichiometric value and themaximum temperature T^(R1) _(max) in reactor R1 between 290 to 350° C.Any adjustment of the flow rate of the main oxygen-containing gas streamwould therefore result in a simultaneous proportional adjustment of theflow rate of the first auxiliary oxygen-containing gas.

The catalyst system of reactor R1 should catalyze the Claus reaction ofH₂S with sulfur dioxide (SO₂) the hydrolysis of COS and CS₂ andoptionally the direct oxidation of H₂S with oxygen to sulfur.

Preferred catalyst used in the catalyst system of reactor R1 is titaniumdioxide (TiO₂), but other usual catalysts, in particular Al₂O₃, cobaltmolybdenum and/or nickel molybdenum can also be used. A further suitablecatalyst is iron, but better results are achieved with titanium dioxide,cobalt molybdenum and nickel molybdenum, in particular with titaniumdioxide.

Smartsulf in Reactor R1

However, since catalysts suitable for Claus reaction of H₂S, hydrolysisof COS and CS₂ and optionally direct oxidation of H₂S are quiteexpensive, it may be desirable that reactor R1 be separated into twocatalytic sections. This embodiment is known under the term SMARTSULF™reactor.

Therefore, in a preferred embodiment, the reactor R1 is composed of twocatalytic sections:

-   -   a first section containing a first catalyst suitable hydrolysis        of COS and/or CS₂ and optionally for direct oxidation of H₂S,        preferably titanium dioxide (TiO₂), operated as an adiabatic bed        without cooling at a maximum temperature T^(R1) _(max), and    -   a second section containing a second catalyst suitable for Claus        reaction of H₂S, preferably Al₂O₃, operating as a        pseudo-isotherm bed with an internal heat exchanger where the        outlet temperature T^(R1) _(o) is not higher and preferably        lower than T^(R1) _(max) but is higher than the dew point of the        sulfur.

In this embodiment, the first section of the first reactor contains afirst catalyst suitable for direct oxidation of H₂S and/or hydrolysis ofCOS and/or CS₂ as previously described and no heat exchanger and isoperated as an adiabatic bed without cooling. Here the temperature iskept at the maximum temperature T^(R1) _(max) in the first section ofthe first reactor, and at this temperature the selective directoxidation of hydrogen sulfide with oxygen is conducted in the presenceof the catalyst contained in the adiabatic bed (as well as thehydrolysis of COS and CO₂).

After the reaction took place in the adiabatic bed, the gas streamcontaining remaining hydrogen sulfide+elemental sulfur+water+sulfurdioxide is then transferred to the second section of the first reactor.In the second section of the first reactor, a different catalyst ispresent than in the first section which catalyzes only the Clausreaction.

The second section of the first reactor contains a second catalystsuitable for Claus reaction of H₂S and means for heating or cooling thegas (a heat exchanger). The outlet temperature of the second section ofthe first reactor is kept at a temperature T^(R1) _(o) which is nothigher and preferably lower than T^(R1) _(max). T^(R1) _(o) ispreferably below 290° C. but is higher than the dew point of the sulfur.In the second section of the first reactor the Claus Eq. 1 reaction:

2H₂S+SO₂

3/xS_(x)+2H₂O  Eq. 1

occurs. This reaction is an equilibrium reaction, and the equilibrium isshifted to the side of the elemental sulfur the lower the temperatureis. The outlet temperature T^(R1) _(o) of the second section of thefirst reactor is kept above the dew point of the elemental sulfur, andthus, the equilibrium is not sufficiently shifted to the side of theelemental sulfur, but the sulfur is kept in gaseous form and thus doesnot deactivate the catalyst.

The dew point of the elemental sulfur decreases with the sulfurconcentration in the gas. In the first section of the first reactor, thedew point of the elemental sulfur is generally between 220° C. and 250°C. Therefore, the outlet temperature T^(R1) _(o) of the second sectionof the first reactor is preferably superior to the dew point ofelemental sulfur but not greater than 290° C., for example 220°C.≦T^(R1) _(o)≦250° C. Preferably T^(R1) _(o) is 1° C. to 20° C. abovethe sulfur dew point in reactor R1, preferably 5° C. to 10° C. above thesulfur dew point.

The second catalyst of reactor R1 contains a Claus catalyst that onlycatalyses the Claus reaction. Any known Claus catalyst, such as Al₂O₃ orTiO₂ can be used.

A main advantage of separating reactor R1 into two catalytic sections isto ensure the maximum hydrolysis of COS and CS₂ in the first sectionwhile improving the Claus reaction in the second section since the Clausreaction is favored at lower temperatures. This configuration of thereactors makes it possible to use less reactors in the overall process,preferably only 2 serially connected catalytic reactors to obtain a veryhigh hydrogen sulfide removal, for example the sulfur recoveryefficiency is more than 97 vol. % of H₂S, based on the initial amount ofH₂S present in the H₂S-containing gas stream treated by the process ofthe invention.

Step e

The claimed process further includes a step e) where the gas streamdepleted in H₂S leaving the reactor R1 is cooled by passing through acondenser where liquid sulfur is condensed and withdrawn and a gasstream depleted in sulfur is recovered.

A gas stream depleted in sulfur is thus obtained, wherein preferably 80to 95 vol. % of the H₂S contained in the H₂S-containing gas streamentering the process is converted.

Step f: Optional Other Reactors in Series

Succession of catalytic reactors associated with optional reheaters andsulfur condensers can be used to increase sulfur recovery.

The gas stream depleted in sulfur but still containing residual amountsof hydrogen sulfide and sulfur dioxide obtained from reactor R1 thenoptionally passes through a series of reactors, preferably 1 or 2,containing a catalyst system which catalyzes the Claus reaction of H₂Swith sulfur dioxide (SO₂) before reaching the last reactor R of theplant.

The temperature of the catalytic reactors is decreased with eachadditional reactor, but never below the sulfur dew point (about 220 to250° C. at operating pressures) to avoid sulfur condensation insidereactors and possible catalyst deactivation. Lower temperature decreasesClaus reaction kinetics, but allows Claus reaction to perform highersulfur recovery rate.

Typical temperatures found at the outlet of second and third catalyticClaus reactors are about 240 and 200° C. respectively.

In this optional embodiment, an auxiliary oxygen-containing gas streamcan be supplemented to the gas stream introduced in each reactor, andthese reactors can contain a catalyst which performs both the directoxidation of H₂S with oxygen to sulfur and the Claus reaction of H₂Swith sulfur dioxide (SO₂). The gas stream depleted in H₂S obtained atthe exit of each reactor can be further transferred to a sulfurcondenser to obtain a gas stream depleted in sulfur.

Step g

The process of the invention involves at least two serially connectedreactors. The gas stream depleted in sulfur obtained from reactor R1passes to the last reactor R together with a last auxiliaryoxygen-containing gas stream.

The gas stream depleted in sulfur obtained in step e) or optionally instep f) may further pass through a heater located between the condenserof step e) or f) and the reactor R of step g).

The last reactor R contains a catalyst system which catalyzes both thedirect oxidation of H₂S with oxygen to sulfur and the Claus reaction ofH₂S with sulfur dioxide (SO₂), said reactor R being operated at amaximum temperature T^(R) _(max) below the maximum temperature T^(R1)_(max) of reactor R1.

Within the meaning of the invention, the last reactor is the last inwhich H₂S-containing gas stream enters in the process.

The last reactor R should maximize the conversion of H₂S through Clausreaction so that the overall process of the invention efficientlyremoves H₂S.

It is however necessary to limit the amount of oxygen supplemented inthe last reactor R since the addition of oxygen favors direct oxidationof H₂S which, as previously indicated, is an exothermic reaction.However, the Claus reaction which should be maximized in the lastreactor R is favored at lower temperatures and would therefore beaffected by a rise in temperature due to direct oxidation of H₂S.

Therefore, the flow rate of oxygen in the last auxiliaryoxygen-containing gas stream should only represent 0.1 to 4 vol. % ofthe total flow rate of the oxygen-supplemented in the process,preferably 0.1 to 2 vol. %, and more preferably 0.5 to 1.5 vol. % of thetotal flow rate of the oxygen supplemented in the process.

It can be noted that the last auxiliary oxygen-containing gas streamrepresents a negligible part of the total oxygen-containing gas streamsupplemented in the process of the invention. This makes it possible toobtain faster O₂-adjustment in the last reactor since only a smallamount of oxygen is required at this stage (supplementation of higheramounts of oxygen would require bigger valves, an thus longer reactiontime of the system). This contributes to obtaining an increasedreactivity of the system to supply the right amount of oxygen to theoverall process, and this leads to an increased global accuracy of theoxygen demand in the process of the invention compared to a prior artprocesses supplementing oxygen only at the entrance of the processleading to greater sulfur recovery.

The flow rate of oxygen in the last auxiliary oxygen-containing gasstream, while always representing 0.1 to 4 vol. % of the total flow rateof the oxygen-containing gas stream supplemented in the process shouldpreferably be adjusted to produce more or less SO₂ in reactor R in orderto ensure that the volumetric ratio of H₂S/SO₂ at the exit of the lastreactor R remains. from 1.9 to 2.2.

In a preferred embodiment of the present invention, in order to maximizethe amount of H₂S removal from the claimed process, the volumetric ratioof H₂S/SO₂ at the exit of the last reactor R is maintained from 1.9 to2.2 during the whole process.

The flow rate of oxygen in the last auxiliary oxygen-containing gasstream is increased when the value of the volumetric ratio of H₂S/SO₂measured in step i) is above 2, and is decreased when the volumetricratio of H₂S/SO₂ measured in step i) is below 2.0.

Since it is not possible to remove 100% of the H₂S contained in the acidgas fed to the process, there is always remaining H₂S to measure thevolumetric ratio of H₂S/SO₂ at the exit of the last reactor R.

If the oxygen demand in the last auxiliary oxygen-containing gas streamis higher than 2.5 vol. % of the total flow rate of the oxygensupplemented in the process, in particular from 2.8 to 4 vol. %,preferably from 3 to 3.6 vol. %, a signal can be sent to the mainoxygen-containing gas stream to increase the flow rate of oxygen in themain oxygen-containing gas stream in proportion.

If the oxygen demand in the last auxiliary oxygen-containing gas streamis lower than 1.5 vol. %, of the total flow rate of the oxygensupplemented in the process, in particular from 0.1 to 1.5 vol. %,preferably from 0.4 to 1.2 vol. %, a signal can be sent to the mainoxygen-containing gas stream to decrease the flow rate of oxygen in themain oxygen-containing gas stream in proportion.

Therefore, in the preferred embodiment previously disclosed where theflow rate of the first auxiliary oxygen-containing gas stream isproportional to the flow rate of the main auxiliary oxygen-containinggas stream of a proportionality factor (b) and the flow rate of the mainauxiliary oxygen-containing gas stream is set to a fixed value, thisfixed value of the flow rate of the main auxiliary oxygen-containing gasstream will however vary if the oxygen demand is higher than the maximalinstruction range for the flow rate of the last auxiliaryoxygen-containing gas stream.

The catalyst system of reactor R should catalyze the Claus reaction ofH₂S with sulfur dioxide (SO₂)

As already disclosed, suitable catalyst for the Claus reaction can beany known Claus catalyst, for example selected from titanium dioxide(TiO₂), cobalt molybdenum, nickel molybdenum, iron and/or Al₂O₃,preferably titanium dioxide (TiO₂).

The reactor R is operated at a maximum temperature T^(R) _(max) belowT^(R1) ₁, and preferably the maximum temperature in the last reactor isranging from 180 to 240° C., preferably 190 to 210° C.

Smartsulf in Reactor R

In a preferred embodiment, as previously described for reactor R1, thereactor R can be composed of two catalytic sections:

-   -   a first section containing a first catalyst suitable for direct        oxidation of H₂S, preferably titanium dioxide (TiO₂), operated        as an adiabatic bed without cooling at a maximum temperature        T^(R) _(max) ranging from 180 to 240° C., preferably 190 to 210°        C., and    -   a second section containing a second catalyst suitable for Claus        reaction of H₂S, preferably Al₂O₃, operating as a        pseudo-isotherm bed with an internal heat exchanger where the        outlet temperature T^(R) _(o) is higher than water dew point and        lower than sulfur dew point, preferably ranging from 105 to 140°        C., and more preferably 110 to 125° C.

In this embodiment, the first section of the last reactor contains afirst catalyst suitable for direct oxidation of H₂S as described forreactor R1 and no heat exchanger and is operated as an adiabatic bedwithout cooling. The temperature in this first section of reactor R iskept at the maximum temperature T^(R) _(max) ranging from 180 to 240°C., preferably 190 to 210° C.

It should be noted that catalysts that require an oxygen surplus cannotbe considered suitable catalysts for direct oxidation of H₂S since theyrequire residual free oxygen available downstream the catalyst whichwould render any downstream control of the H₂S/SO₂ ratio useless.

After the reaction took place in the adiabatic bed, the gas streamcontaining remaining hydrogen sulfide+elemental sulfur+water+sulfurdioxide is then transferred to the second section of the last reactor.The second section of the last reactor contains a catalyst whichcatalyzes only the Claus reaction and means for heating or cooling thegas (a heat exchanger). The outlet temperature T^(R) _(o) is higher thanwater dew point and lower than sulfur dew point in reactor R in order tocondensate the elemental sulfur while avoiding simultaneous watercondensation.

The dew point of the elemental sulfur decreases with the sulfurconcentration in the gas. Considering that at the outlet of the lastreactor R the concentration of sulfur is already low, at that part ofthe last reactor the dew point of elemental sulfur in reactor R is about170° C. Thus, preferably the outlet temperature T^(R) _(o) of the secondsection of the last reactor is ranging from 105 to 140° C., preferably110 to 125° C.

In a preferred embodiment, the catalytic systems of both reactors R1 andR are separated into two catalytic sections. With this preferredembodiment, a very high hydrogen sulfide removal, for example more than99.8 vol. % of H₂S, based on the initial amount of H₂S present in theH₂S-containing gas stream treated by the process of the invention can beobtained with minimal installation costs. When optional Claus reactorsare added in series between reactors R1 and R, they preferably contain acatalytic system separated into two catalytic sections.

Regeneration of the Catalyst

One disadvantage of operating the last reactor at such a low temperatureis that the liquid or solid sulfur deposits on the catalyst andaccumulates. Over time this leads to a deactivation of the catalyst. Thegas leaving the second reactor is essentially free of hydrogen sulfideand can be further used or processed. After some time of operation thecatalyst of the last reactor R is contaminated by liquid and/or solidelemental sulfur to such a degree that it can no longer sufficientlycatalyze the Claus reaction.

In such situation, the operating conditions between the seriallyconnected reactors are switched, and the gas flow is also switched sothat the previous last reactor R is operated in the conditions ofprevious reactor R1, and the previous first reactor R1 is operated inthe conditions of previous reactor R. Thus, now the previous lastreactor R is operated at the maximum temperature T^(R1) _(max) and atthe outlet temperature T^(R1) _(o) previously defined for R1, and theprevious first reactor R1 is operated at the temperatures T^(R) _(max)and T^(R) _(o). The gas streams are also switched so that the gas streamto be desulfurized is now transferred to the previous last reactor R.Accordingly, the previous first reactor is now operated at thetemperatures of the previous last reactor and thus acts in the same wayas the previous last reactor. Essentially, by switching the operationconditions and the gas flow, the previous last reactor now becomes thefirst reactor, and the previous first reactor now becomes the lastreactor. The elemental sulfur deposited on the catalyst in the previouslast reactor is desorbed at the new temperatures of operation and leavesthe previous last reactor essentially with the gas stream which istransferred to the sulfur condenser.

The switch is repeated when the catalyst in the “new” last reactor isinactivated by the deposited sulfur.

The switching process of the gases between the first and the lastreactor can be done by usual and known distributors. Preferred devicesfor effecting the switching process are disclosed and described in DE 102010 034 070, the content of which is included herein by reference.

When high sulfur recovery rate is sought, the time necessary toswitch-over the sulfur loaded reactor R into the position of the reactorR1 and vice versa can lead to an important loss in sulfur recovery dueto the sudden change of operating conditions. In addition, during thisswitch-over, the air demand of the reactors necessarily varies. Thisleads to a transitory period where the sulfur recovery rate decreases.According to the invention, however, the actual air demand of the lastreactor R can be adjusted within seconds after the switch of reactorspositions. This reduces the duration of the transitory period andensures that even during the switch of reactors positions, the sulfurrecovery rate remains constantly high.

In particular, the volumetric ratio of H₂S/SO₂ at the exit of the newlast catalytic reactor R reaches the desired value from 1.9 and 2.2within 1 seconds to 2 minutes during the whole process and in particularafter the switch of the reactors by adjustment of the flow rate of thelast auxiliary oxygen-containing gas stream.

Step h

The claimed process can optionally further includes a step h) where thegas stream depleted in H₂S leaving the last reactor R is cooled bypassing through a condenser where liquid sulfur is condensed andwithdrawn and a gas stream depleted in sulfur is recovered. If reactor Ris operated below the sulfur dew point, as in the SMARTSULF™ embodimentpreviously described, no downstream condenser is needed, as this wouldnot increase the sulfur recovery rate.

Thanks to the process of the invention, the sulfur recovery efficiencyis above 99 vol. %, more preferably above 99.5 vol %, and even morepreferably up to 99.8 vol. % of H₂S or above, based on the initialamount of H₂S present in the H₂S-containing gas stream.

In a preferred embodiment of the invention, the process contains twoserially connected reactors, each composed of two catalytic sections:the first section suitable for direct oxidation of H₂S and the secondsection suitable for Claus reaction, wherein the operating conditionsbetween the two reactors can be switched in order to ensure regenerationof the catalyst.

Step i)

The claimed process further includes a step i) of measurement of thevolumetric ratio of H₂S/SO₂ at the exit of the last catalytic reactor R.

The volumetric ratio of H₂S/SO₂ at the exit of the last reactor R can bemeasured by well-known Air Demand Analyzers, also called ADA.

By controlling that the volumetric ratio of H₂S/SO₂ at the exit of theclaimed process remains around the stoichiometry of the Claus reactionof 2, in particular from 1.9 and 2.2, the conversion of H₂S can befurther improved. As previously explained, the inventors haveunexpectedly found that by supplementing the last reactor of the processwith a last auxiliary oxygen-containing gas stream in a specific flowrate of oxygen, it was possible to control in a precise and veryreactive manner the volumetric ratio of H₂S/SO₂ at the exit of the lastreactor R of the claimed process.

Therefore, thanks to the specific and sensitive control of the oxygensupplemented in the different steps of the claimed process, it ispossible to provide a high desulfurization that is stable over time, andto easily compensate any variation in the composition of theH₂S-containing gas stream. Indeed, any variation of the volumetric ratioof H₂S/SO₂ from the stoichiometry detected at the exit of the lastreactor R would immediately be compensated by an adjustment of theoxygen supplemented at the entry of the last reactor. In addition, ifthe oxygen demand at the entry of the last reactor is higher that themaximal instruction range set for the process, an increase of the flowrate of the main oxygen-containing will be ordered. In the same manner,if the oxygen demand at the entry of the last reactor is lower that theminimal instruction range set for the process, a decrease of the flowrate of the main oxygen-containing will be ordered. This oxygensupplement control ensures that the Claus reaction is operated in thebest conditions with a minimal delay in the adjustment regarding theoxygen demand, thus maximizing the H₂S removal.

The present invention also encompasses a method for controlling thevolumetric ratio of H₂S/SO₂ at the exit of a sulfur recovery unit withthe process described above.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, a process involving the oxygen controlaccording to the invention in a SMARTSULF™ preferred embodiment isillustrated.

The H₂S-containing gas stream (line 1) is mixed with a mainoxygen-containing gas stream (line 2) and introduced in a furnace (3)without catalyst.

The H₂S flow rate in the feed gas is measured and the flow rate of themain oxygen-containing gas stream sent to the furnace is controlled inproportion to this value. The content of H₂S in the feed gas is measuredby an Analysis Indicator Control QIC (32 in FIG. 1) as well as the flowrate of the H₂S-containing gas stream (not shown), which gives the flowrate of H₂S introduced in the furnace. The flow rate of the mainoxygen-containing gas stream sent to the furnace is controlled by themain air valve in proportion to the H₂S flow rate.

SO2 is produced by the reaction 2H2S+3O2→2SO2+2H2O.

The stream of gas leaving the furnace thus contains SO₂, remaining H₂S,and impurities generated in the furnace such as COS, CS₂ . . . .

The stream is cooled by passing through a condenser (4) where liquidsulfur is condensed and withdrawn (line 5), and the stream of gas isrecovered at the top of the condenser (line 6) at a temperature of about130° C. The sulfur removed corresponds to 50-70% of the sulfur presentinitially in the acid gases.

The recovered stream of gas is reheated in one or more heater (7) andoptionally mixed with a first auxiliary oxygen-containing gas stream(through valve 36 and line 30) before entering the first reactor (8).This first reactor (8) is filled with titanium oxide or another suitablecatalyst bed (9) which catalyzes both the Claus reaction of H₂S withsulfur dioxide (SO₂), the hydrolysis of COS and CS₂ and optionally thedirect oxidation of H₂S with oxygen to sulfur. Usually the temperatureof the first reactor (8) reaches 315 to 330° C. which is of particularinterest to better achieve the hydrolysis of COS and CS₂ which isimproved at such high temperature.

The first auxiliary oxygen-containing gas stream sent to the firstcatalytic reactor (through valve 36 and line 30) is controlled with themaximal temperature reached in the reactor (350° C.) through theTemperature Indicator Control (TIC) device which controls the opening ofvalve (36). Indeed, residual H₂S oxidizes with oxygen coming from line30 when contacted with the TiO₂ based catalyst in reactor (8). Thisreaction is exothermic and results in an increase of the reactor's (8)temperature. Sufficiently high temperatures can be obtained thuspermitting COS and CS2 hydrolysis, and this is of particular interest ifthe heater (7) is unable to provide high enough temperature in a simpleand economic manner.

The separation of the catalytic system of reactor (8) into two sections(SMARTSULF™ reactor) is of particular interest in this configuration. Inthis embodiment, the first adiabatic area (8A) of the reactor can beoperated at high temperature (290-340° C.) to enhance previously saidhydrolysis, and the second pseudo-isotherm area (8B) can be operated atmuch lower temperature (200-280° C.) to improve sulfur recovery ratethrough Claus reaction. An external or internal heat exchanger(thermoplates for example) ensures the cooling of the second area whichbehaves as pseudo-isotherm.

Depending on the maximal acceptable sulfur residual concentration, extracatalytic reactors can be added in order to decrease the H₂Sconcentration in the treated vapor effluent (not shown on the figure).

The stream of gas leaving the first reactor (8) containing SO₂ andremaining H₂S is cooled by passing through a condenser (11) and a sulfurtrap (12) where liquid sulfur is condensed and withdrawn (line 13), andthe stream of gas is recovered at the top of the condenser (line 14) ata temperature of about 130° C. The sulfur removed corresponds to 80 to95 vol. % of H₂S, based on the initial amount of H₂S present in theH₂S-containing gas stream treated.

The recovered stream of gas is reheated in one or more heater (15) andmixed with a last auxiliary oxygen-containing gas stream (through valve37 and line 31) before entering the last reactor. This last reactor (16)is filled with titanium oxide or another suitable catalyst bed (17)which catalyzes both the direct oxidation of H₂S with oxygen to sulfurand the Claus reaction of H₂S with sulfur dioxide (SO₂).

The separation of the catalytic system of reactor (16) into two sections(SMARTSULF™) is of particular interest in this configuration. In thisembodiment, the first adiabatic area (16A) of the reactor can beoperated at a temperature ranging from 180 to 240° C., and the secondpseudo-isotherm area (16B) can be operated at much lower temperature(105 to 140° C.) to improve sulfur recovery rate through Claus reaction.An external or internal heat exchanger (thermoplates for example)ensures the cooling of the second area which behaves as pseudo-isotherm.

The volumetric ratio of H₂S/SO₂ at the exit of the last reactor R ismeasured by well-known Air Demand Analyzers, also called ADA (33 in FIG.1). Deviation from the stoichiometric value, i.e from the instructionrange of H₂S/SO₂ volumetric ratio of 1.9 to 2.2, is rectified by asignal to the air valve (37) which will adjust the flow rate of the lastauxiliary oxygen-containing gas.

Since the last auxiliary oxygen-containing gas stream is much smallerthan the main oxygen-containing gas stream it can react a lot faster andthus allows a much more precise control of the H₂S/SO₂ ratio.

As previously indicated, the specific distribution and control of theoxygen supplemented in the claimed process improved the sulfur recoveryrate of a conventional Claus unit substantially. Additionally, thelong-term average values were also improved.

Downstream effluent (line 18) can be cooled by passing through acondenser where liquid sulfur (not shown) is condensed and then theeffluent is withdrawn (line 22).

It is conventional to separate the sulfur which leaves the reactor ingaseous form in a downstream condenser. According to anotherconfiguration of the invention illustrated in FIG. 1, a common sulfurcondenser can be used for each two reactors by using a multiway valve(21) being installed between a first reactor and the downstream sulfurcondenser. This means that the installed sulfur condenser is alwaysflowed through in the same direction regardless of the position of thereactors.

It is possible to easily regenerate the catalyst in the process of theinvention. To do so, two 4 way valves (20-21) are connected to theentrance and the exit of both SMARTSULF™ reactors, and allow to switchthe position of the reactors. There is in this configuration, mostpreferably a unique condenser (12) to collect liquid sulfur. The firstreactor is working above the sulfur dew point and needs the condenser(12) to collect sulfur as liquid element. Then the last reactor isworking at sub dew point, to be able to form sulfur from lower H₂S andSO₂ partial pressures. This last reactor accumulates liquid sulfur whichcondenses on the catalyst, thus after some time plugging the process.Liquid sulfur condensed on the catalyst needs to be evaporated (warm-upof the reactor) to allow the catalyst being fully regenerated. This stepis done by switching the position of the two reactors together with theinternal cooling. Directly after the switching the temperature of theprevious last (cold) reactor is increased, allowing the liquid sulfur toevaporate and to be further recovered after being cooled down in thecondenser (12).

Referring now to FIG. 2, a process involving the oxygen controlaccording to the invention in a classic Claus Unit (without SMARTSULF™reactor) is illustrated.

In this embodiment, the difference with FIG. 1 is that both catalyticreactors (8) and (16) are adiabatic and contain no internal heatexchanger to control the temperature. The temperature in the firstreactor (8) is therefore between 290-350° C. at the outlet, and thetemperature in the last reactor (16) is between 180-240° C. at theoutlet.

In addition, in this embodiment, it is not possible to regenerate thecatalyst in the process of the invention since no 4-way valves (20-21)are connected to the entrance and the exit of the reactors. Therefore,the reactors should not operate below the dew point of elementary sulfurto avoid sulfur condensation on the catalyst and thus, plugging of thewhole process.

The control of the oxygen distribution in this classic Claus unitprovides better desulfurization than what would be obtained in the sameunit without oxygen supplement.

However, the process of the invention is operated in the best conditionsin the preferred embodiment illustrated in FIG. 1, thus maximizing theH₂S removal.

1. A process for the removal of hydrogen sulfide (H₂S) from aH₂S-containing gas stream through two or more serially connectedcatalytic reactors, which process comprises: a) mixing theH₂S-containing gas stream with a main oxygen-containing gas stream toobtain a gas stream containing both H₂S and oxygen, b) introducing theobtained gas stream containing both H₂S and oxygen into a furnacewhereby a gas stream depleted in H₂S is obtained, c) transferring thegas stream depleted in H₂S to a sulfur condenser to obtain a gas streamdepleted in sulfur, d) introducing the gas stream depleted in sulfur,optionally together with a first auxiliary oxygen-containing gas stream,into a first catalytic reactor R1 containing a catalyst system whichcatalyzes the Claus reaction of H₂S with sulfur dioxide (SO₂), thehydrolysis of COS and CS₂ and optionally direct oxidation of H₂S withoxygen to sulfur, said reactor being operated at a maximum temperatureT^(R1) _(max) between 290 and 350° C., whereby a gas stream depleted inH₂S is obtained, e) transferring the gas stream depleted in H₂S to asulfur condenser to obtain a gas stream depleted in sulfur, f)optionally introducing the gas stream depleted in sulfur obtained fromreactor R1 through a series of reactors and condensers, preferably 1 or2, each reactor containing a catalyst system which catalyzes the Clausreaction of H₂S with sulfur dioxide (SO₂) before reaching the lastreactor R of the process, g) introducing the gas stream depleted insulfur together with a last auxiliary oxygen-containing gas stream intothe last catalytic reactor R containing a catalyst system whichcatalyzes the Claus reaction of H₂S with sulfur dioxide (SO₂) and thedirect oxidation of H₂S with oxygen to sulfur, said reactor beingoperated at a maximum temperature T^(R) _(max) below the maximumtemperature T^(R1) _(max) of reactor R1, whereby a gas stream depletedin H₂S is obtained, h) optionally transferring the gas stream depletedin H₂S to a sulfur condenser to obtain a gas stream depleted in sulfur,i) measuring the volumetric ratio of H₂S/SO₂ at the exit of the lastcatalytic reactor R, wherein the flow rate of oxygen in the mainoxygen-containing gas stream and in the optional auxiliaryoxygen-containing gas streams represents 96 to 99.9 vol. % of the totalflow rate of the oxygen supplemented in the process, preferably 98 to99.8 vol. %, and more preferably 98.5 to 99.5 vol % the flow rate ofoxygen in the last auxiliary oxygen-containing gas stream represents 0.1to 4 vol. % of the total flow rate of the oxygen supplemented in theprocess, preferably 0.1 to 2 vol. %, and more preferably 0.5 to 1.5 vol.% and wherein the flow rate of oxygen in the last auxiliaryoxygen-containing gas stream is adjusted depending on the value of thevolumetric ratio of H₂S/SO₂ measured at the exit of the last catalyticreactor R in step i) so that the volumetric ratio of H₂S/SO₂ measured instep i) remains between 1.9 and 2.2.
 2. Process according to claim 1,wherein the flow rate of oxygen in the last auxiliary oxygen-containinggas stream is increased when the value of the volumetric ratio ofH₂S/SO₂ measured in step i) is above 2, and is decreased when thevolumetric ratio of H₂S/SO₂ measured in step i) is below 2.0.
 3. Processaccording to claim 1, wherein, in step a), the flow rate of oxygen inthe main oxygen-containing gas stream and in the optional auxiliaryoxygen-containing gas streams is calculated so that the volumetric ratioof H₂S in the H₂S-containing gas stream/O₂ in the oxygen-containing gasstream be above the stoichiometric value of the reactions operated inthe furnace of 2, in particular between 2.002 to 2.5, preferably 2.002to 2.2, more preferably 2.002 to 2.08.
 4. Process according to claim 3,wherein the volumetric ratio of H₂S in the H₂S-containing gas stream/O₂in the main oxygen-containing gas stream is maintained above thestoichiometric value of the reactions operated in the furnace of 2during the whole process.
 5. Process according to claim 1, wherein thefurnace is operated at a temperature of 900° C. to 1400° C., morepreferably 1100° C. to 1300° C.
 6. Process according to claim 1, whereinthe gas stream depleted in sulfur obtained in step c) further passesthrough a heater located between the condenser of step c) and thereactor R1 of step d).
 7. Process according to claim 1, wherein the flowrate of oxygen in the first auxiliary oxygen-containing gas stream isadjusted to ensure that the maximum temperature T^(R1) _(max) in reactorR1 remains between 290 to 350° C., preferably 310 to 340° C., and morepreferably 315 to 330° C.
 8. Process according to claim 7, wherein thetemperature T^(R1) _(max) in reactor R1 is maintained between 290 to350° C., preferably 310 to 340° C., and more preferably 315 to 330° C.during the whole process.
 9. Process according to claim 1, wherein thecatalyst system of reactors R1 and R comprises at least one catalystselected from titanium dioxide (TiO₂), cobalt molybdenum, nickelmolybdenum, iron and/or Al₂O₃, preferably titanium dioxide (TiO₂). 10.Process according to claim 1, wherein the reactor R1 is composed of twocatalytic sections: a first section containing a first catalyst suitablefor direct oxidation of H₂S and/or hydrolysis of COS and/or CS₂,preferably titanium dioxide (TiO₂), operated as an adiabatic bed withoutcooling at a maximum temperature T^(R1) _(max), and a second sectioncontaining a second catalyst suitable for Claus reaction of H₂S,preferably Al₂O₃, operating as a pseudo-isotherm bed with an internalheat exchanger where the outlet temperature T^(R1) _(o) is not higherand preferably lower than T^(R1) _(max) but is higher than the dew pointof the sulfur.
 11. Process according to claim 1, wherein the gas streamdepleted in sulfur obtained in step e) further passes through a heaterlocated between the condenser of step e) or f) and the reactor R of stepg).
 12. Process according to claim 1, wherein the volumetric ratio ofH₂S/SO₂ at the exit of the last reactor R is maintained from 1.9 to 2.2during the whole process.
 13. Process according to claim 1, wherein thereactor R is composed of two catalytic sections: a first sectioncontaining a first catalyst suitable for direct oxidation of H₂S,preferably titanium dioxide (TiO₂), operated as an adiabatic bed withoutcooling at a maximum temperature T^(R) _(max) ranging from 180 to 240°C., preferably 190 to 210° C., and a second section containing a secondcatalyst suitable for Claus reaction of H₂S, preferably Al₂O₃, operatingas a pseudo-isotherm bed with an internal heat exchanger where theoutlet temperature T^(R) _(o) is higher than water dew point and lowerthan sulfur dew point, preferably ranging from 105 to 140° C., and morepreferably 110 to 125° C.
 14. Process according to claim 1, wherein theoperating conditions between the serially connected reactors areswitched, and the gas flow is also switched so that the previous lastreactor R is operated in the conditions of previous reactor R1, thusbecoming new reactor R1, and the previous first reactor R1 is operatedin the conditions of previous reactor R, thus becoming new reactor R.15. Process according to claim 1, wherein the volumetric ratio ofH₂S/SO₂ at the exit of the new last catalytic reactor R reaches thedesired value between 1.9 and 2.2 within 1 seconds to 2 minutes duringthe whole process and in particular after the switch of the reactors byadjustment of the flow rate of the last auxiliary oxygen-containing gasstream.
 16. Process according to claim 1, wherein the sulfur recoveryefficiency is above 99 vol. %, more preferably above 99.5 vol %, andeven more preferably up to 99.8 vol. % of H₂S or above, based on theinitial amount of H₂S present in the H₂S-containing gas stream treated.17. Process according to claim 1, wherein if the oxygen demand in thelast auxiliary oxygen-containing gas stream is higher than 2.5 vol. % ofthe total flow rate of the oxygen supplemented in the process, inparticular from 2.8 to 4 vol. %, preferably from 3 to 3.6 vol. %, asignal is sent to the main oxygen-containing gas stream to increase theflow rate of oxygen in the main oxygen-containing gas stream inproportion.
 18. Process according to claim 1, wherein if the oxygendemand in the last auxiliary oxygen-containing gas stream is lower than1.5 vol. %, of the total flow rate of the oxygen supplemented in theprocess, in particular from 0.1 to 1.5 vol. %, preferably from 0.4 to1.2 vol. %, a signal is sent to the main oxygen-containing gas stream todecrease the flow rate of oxygen in the main oxygen-containing gasstream in proportion.